Counterion-Induced Transformations of Cationic Surfactant Micelles Studied by Using the Displacing Effect of Solvatochromic Pyridinium N-Phenolate Betaine Dyes

Abstract

In this paper, we demonstrate that the behavior of a set of eight large-sized negatively solvatochromic pyridinium N-phenolate betaine dyes reflects the principle transformations, occurring in aqueous micellar solutions of three cationic surfactants. As surfactants, cetyltrimethylammonium bromide (CTAB), n-octadecyltrimethylammonium chloride (OTAC), and N-cetylpyridinium bromide (CPB) were used. Normally, for such probes coupled with micelles, a red shift of the vis absorption band is expected as a result of a hydrophobization ("drying") of the micellar interface. However, under addition of electrolytes with anions such as tosylate, salicylate, and some n-alkanesulfonates or n-alkanecarboxylates to the micellar solutions, an unexpected effect was observed. Instead of a red shift, a blue shift of the vis absorption band of some of the dissolved betaine dyes was registered, as compared with the spectrum measured in pure aqueous micellar solutions of CTAB, OTAC, or CPB (Deltalambda(max) up to ca. 80 nm). This blue shift, indicating an increase in the polarity of the dye microenvironment, is explained by displacing the large dye dipoles from the thinned micelles toward the aqueous phase. The effect is well expressed at concentrations of C(betaine dye) approximately 10(-5) M, C(cationic surfactant) approximately 0.001 M, and C(organic anion) approximately 0.01 M. Transmission electron microscopy of dried samples confirms the distinct changes occurring in the studied micellar systems upon the addition of organic anions. The excess of inorganic salts [C(NaBr, KBr, or KCl) = 0.5-4.0 M] restored the position of the vis absorption band or even shifted it toward the red. Moreover, some of the betaine dyes studied (i.e., the more hydrophobic ones) stay in the micellar pseudophase or precipitate under the aforementioned concentration conditions. The peculiarities of the behavior of these betaine dyes are in agreement with their molecular structure.

A Family of Chiral Metal–Organic Frameworks

“…Both metal-organic frame- works are able to adsorb fluorescein (8) and merocyanine 9 from solution. The adsorption behavior differs, however, when Reichardts dye (10), [23] a very large molecule (1.26 1.01 0.81 nm 3 ) [24] is used. Dye 10 was previously employed to probe the enormous pore size in MOF-177 [16] and is completely adsorbed by 6 (see Figure S12 in the Supporting Information).…”

Interfacial properties of cetyltrimethylammonium-coated SiO2 nanoparticles in aqueous media as studied by using different indicator dyes

“…Dissolved in pure solvents, solvent mixtures, polymers, ultramicroheterogeneous media such as micellar solutions of surfactants, vesicles, phospholipids bilayers, etc. [53][54][55], the standard dye (4-(2,4,6-triphenylpyridinium-1-yl)-2,6-diphenylphenolate) exhibits strong negative solvatochromism. A solvent change from polar water (λ max = 453 nm) to micellar solutions of cationic surfactant (e.g., λ max = 546 nm in 0.05 M CTAB solution) leads to a batochromic charge-transfer band shift with decreasing solvent polarity [52].…”

An MD simulation study of Reichardt’s betaines in surfactant micelles: Unlike orientation and solvation of cationic, zwitterionic, and anionic dye species within the pseudophase

“…In order to reveal the consequences of more substantial structural changes, we decided to extend the MD studies with following betaine dyes, also used for studying micellar solutions of surfactants [12]…”